This invention relates to a rotary internal combustion engine. There have been proposed and practically employed a variety of rotary internal combustion engines and in a typical prior art rotary internal combustion engine, as illustrated in FIG. 1 of the accompanying drawings, a rotor 102 is eccentrically mounted on a main shaft 101 which is in turn journalled in a casing 103. In the prior art rotary internal combustion engine, explosive force is applied on the main shaft 101 not only in the rotation direction a of the shaft, but in the opposite direction b to thereby impede the rotation efficiency of the main shaft resulting in excessive consumption of fuel. And the projection 104 formed in an intermediate position on the inner surface of the wall of the casing 103 is easily subjected to wear which leads to damage of the engine. Furthermore, in the prior art rotary internal combustion engine, exhaust gas can not be discharged satisfactorily and since the rotor eccentrically rotates with respect to the main shaft, the rotor is subjected to centrifugal force resulting in undesirable vibration of the rotor.